catena-Poly[[bis(quinolin-8-amine-κ2 N,N′)cadmium(II)]-μ-cyanido-κ2 N:C-[dicyanidonickel(II)]-μ-cyanido-κ2 C:N]

The title compound forms a one-dimensional coordination polymer containing alternating six-coordinate Cd and four-coordinate Ni atoms, linked by bridging cyano ligands

In the title compound, [CdNi(C 9 H 8 N 2 ) 2 (CN) 4 ] n , the Cd and Ni atoms both lie on centres of inversion in space group P2 1 /c. The Cd atom is coordinated by two bidentate quinolin-8-amine ligands and by the N atoms of two cyano ligands, while the square planar Ni atom is coordinated by the C atoms of four cyano ligands. These units form a one-dimensional coordination polymer containing an (-NC-Ni-CN-Cd-) n backbone, and the coordination polymer chains are linked into a three-dimensional array by a combination of N-HÁ Á ÁN and C-HÁ Á ÁN hydrogen bonds, augmented by astacking interaction.

Structure description
Transition-metal coordination compounds in which cyano ligands play the main structure-forming role, so-called cyanocarbanion or cyanometallate complexes, have been the subject of interest for many years, because of their magnetic and luminescent properties (Sieklucka et al., 2011;Benmansour et al., 2007Benmansour et al., , 2008Benmansour et al., , 2009Benmansour et al., , 2012Setifi et al., 2009;Yuste et al., 2009;Lehchili et al., 2017) including, in particular, their spin-crossover behaviour (Benmansour et al., 2010;Setifi et al., 2013, 2014, Bartual-Murgui et al., 2013. In a continuation of our general study of this area, we now report the crystal and molecular structure of the title compound. In the structure of the title compound, the Cd and Ni ions both lie on centres of inversion, selected for convenience as those at (0.5, 0.5, 0.5) and (0.5, 0.5, 0), respectively.

data reports
The [Ni(CN) 4 ] 2À units adopts the usual square planar configuration, while the Cd centre is coordinated by two bidentate quinolin-8-amine units and by the N atoms of two cyano ligands. The structure thus consists of one-dimensional coordination polymer based on an (-NC-Ni-CN-Cd-) n backbone and running parallel to [001]. In the reference chain [Cd{quinolin-8-amine) 2 ] 2+ units centred at (0.5, 0.5, n + 0.5) alternate with [Ni(CN) 4 ] 2À units centred at (0.5, 0.5, n), where n represents an integer in each case (Fig. 1). There are two types of N-HÁ Á ÁN hydrogen bond in the structure (Table 1). Those involving atom H8A lie within the coordination polymer chain, but those involving atom H8B link the chain along (0.5, 0.5, z) to those along (0.5, 0, z) and (0.5, 1, z), so forming a sheet of hydrogen-bonded chains lying parallel to (100) (Fig. 2). Sheets of this type are linked into a threedimensional array by two types of direction-specific interactions, a C-HÁ Á ÁN hydrogen bond (Table 1) and astacking interaction. The C-HÁ Á ÁN hydrogen bond combines with the inversion symmetry at both metal centres to generate a chain running parallel to the [201] direction ( Fig. 3), which links the (100) sheets into a three-dimensional structure. In addition, the carbocyclic rings in the quinolin-8-amine ligands at (x, y, z) and (2 À x, 1 À y, 1 À z), which lie in adjacent (100) sheets, are strictly parallel with an interplanar spacing of 3.4070 (6) Å ; the ring-centroid separation is 3.5856 (8) Å , with a ring-centroid offset of ca 1.117 (2) Å : the interactions between the two types of ring in these two ligands are similar (Fig. 4). Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The coordination polymer formed by the title compound. For the sake of clarity many of the C atom labels have been omitted: the atoms marked with a, b, c, d or e are at the symmetry positions (1 À x, 1 À y, Àz), (1 À x, 1 À y, 1 À z), (x, y, À1 + z), (x, y, 1 + z) and (1 À x, 1 À y, 2 À z), respectively,. Displacement ellipsoids are drawn at the 80% probability level.

Figure 3
Part of the crystal structure showing the formation of a hydrogen-bonded chain running parallel to the [201] direction. For the sake of clarity, the H atoms not involved in the motif shown have been omitted.

Figure 2
A projection along [100] of part of the crystal structure showing the formation of a hydrogen-bonded sheet of polymer chains, lying parallel to (100). For the sake of clarity, the H atoms bonded to C atoms have been omitted.
The structure of the title compound is very similar to that of the iron(II)-nickel analogue, whose structure has been studied at both 293 K and 120 K, where the iron adopts high-spin and low-spin configurations, respectively (Setifi et al., 2014). This structural similarity of the Cd II and Fe II compounds is somewhat unexpected in view of the different effective radii of these ions (Shannon & Prewitt, 1969, 1970, reflected in the differences between the M-N (M = Cd or Fe) distances in the two compounds, typically around 0.30 Å for each type of bond, itself reflected in the difference between the a repeat vectors, 9.4264 (3) Å for M = Cd but only 9.0035 (5) Å for M = Fe at 120 K.

Synthesis and crystallization
A solution of quinolin-8-amine (0.288 g, 2 mmol) in ethanol (10 ml) was added dropwise with stirring at 323 K to a solution of Cd[Ni(CN) 4 ]ÁH 2 O (0.293 g, 1 mmol) in water (10 ml). This mixture was stirred for 4 h at 323 K and then filtered. Slow evaporation of the filtrate over a period of one week, at ambient temperature and in the presence of air, gave crystals suitable for single-crystal X-ray diffraction.

Figure 4
Part of the crystal structure showing the -stacking of quinolin-8-amine ligands. For the sake of clarity, the unit-cell outline and the H atoms have been omitted: the Cd atom marked with an asterisk (*) is at (1.5, 1/2, 1/2).

catena-Poly[[bis(quinolin-8-amine-κ 2 N,N′)cadmium(II)]-µ-cyanido-κ 2 N:C-[dicyanidonickel(II)]-µ-cyanido-κ 2 C:N]
Crystal data [CdNi (C 9  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.